High performance components in aircraft engine turbomachines such as compressor blades bearings, gears, impellers and diffusers are typically coated with a metal compound of titanium, zirconium, chromium or tungsten to improve their wear characteristics and to provide erosion protection. The engine parts are cast or otherwise molded or machined from superalloys, stainless steels, alloy steels or titanium alloys and represent very expensive precision components. Removal of the coating from the underlying base metal is necessary if a defect is discovered in the coating and/or for restoring worn components. It is essential to remove the protective coating from the base metal without suffering any detrimental attack to the underlying base metal.
To selectively strip a metal compound of the group IVB and VIB metals of the periodic table inclusive of: titanium, zirconium, hafnium, chromium, molybdenum and tungsten from a titanium alloy base metal is particularly difficult due to the similarity in high corrosion resistance of both the base metal and coating.
Electrochemical stripping of a metal coating from a metal substrate is well known and is basically the reverse of electrodeposition. A reverse current stripping process is disclosed in U.S. Pat. No. 4,356,069 for removing coatings of chromium and nickel from zinc, steel, aluminum, brass or copper using an aqueous solution of chromic acid, peroxide, sulfuric acid and water. A reverse current stripping process is also taught in U.S. Pat. No. 4,128,463 for stripping a coating of a metal carbide such as tungsten carbide from a titanium or titanium alloy substrate. The composition of the electrolyte comprises an aqueous solution of chromic acid or a chromate ion producing material and optionally a sulfate ion added as sulfuric acid. A method for electrolytically stripping a metal containing refractory coating from a base metal using a caustic electrolyte is taught in U.S. Pat. No.'s 3,151,049 and 4,886,588 respectively.
None of the prior art processes are suitable for stripping a metal coating compound of a group IVB and VIB metal such as, for example, a titanium or zirconium compound from a base metal of titanium or a titanium alloy without attacking the base metal or leaving unwanted corrosion pits on the surface of the base metal. The electrolytic process of the present invention is particularly suited for stripping a titanium or zirconium compound or any group IVB or VIB metal compound from a base metal of a titanium metal or alloy without chemically attacking the base metal or forming corrosion pits in the base metal surface.